It has been known to mount two substrates having filters formed thereon so that the surfaces having the filters formed thereon face each other across an air gap as disclosed in, for example, Japanese Patent Application Publication No. 2007-67617 (referred to as Patent Document 1, hereinafter). Patent Document 1 describes that the two filters are arranged so as to overlap with each other in plan view, and also describes that the two filters are arranged so as not to overlap with each other.